707
Langmuir 1989,5, 707-713
Spectroscopic and Electrochemical Studies of Iodine Coordinated to Noble-Metal Electrode Surfacest G. M. Berry, M. E. Bothwell, B. G. Bravo, G. J. Cali, J. E. Harris, T. Mebrahtu, S. L. Michelhaugh, J. F. Rodriguez, and M. P. Soriaga* Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received December 13, 1988. I n Final Form: March 30, 1989 The kinetics of redox processes of unadsorbed species at noble-metal electrodes are altered to varying degrees by pretreatment of the electrode surface with a full monolayer of iodine. For example, the rate of the quinone/hydroquinone redox couple at pH 4 is increased significantly at Rh, Pd, Ir, and Pt but decreased slightly at Au after these electrodes were coated with iodine. These observations provided the motivation to pursue a comparative study of the surface electrochemical properties of iodine coordinated to these metals and their bimetallic alloys. Experimental measurements were based upon thin-layer electrochemistry, low-energy electron diffraction, Auger electron spectroscopy,and X-ray photoelectron spectroscopy. The findings accumulated to date indicate the following: (i) Iodine is spontaneously and oxidatively chemisorbed as iodine atoms on these metals. (ii) I is covalently bonded to the surface metal atoms; that is, only little or no ionic character exists in the I-metal chemical bond. (iii) Adsorbateadsorbate interactions within the close-packed I layer are negligible with respect to the I-metal bond. (iv) I can be reductively eliminated from the surface either by exposure to electrogeneratedhydrogen or by application of sufficiently negative potentials. (iv) The surface binding strength of I at the subject electrodes decreases in the order Ag > Au > Pt > Ir. (v) The close-packed I layer is not insulating; electron transfer can occur directly from the I adatom. (vi) The conductivity of an I-coated surface is slightly lower than that for a surface which does not contain any chemisorbed material. (vii) The profound dependence of the surface electrochemical properties of I on the surface composition of the metal electrodes makes it a suitable electrochemical tracer in the study of mixed-metal interfaces.
Introduction It is now well established that the interaction of inorganic and organic compounds with transition-metal surfaces has several commonalities with the bonding of such ligands with metal centers in homogeneous or metal-cluster The analogies are rather prominent under electrochemical conditions where the metal surface is initially covered by weakly coordinating electrolyte or solvent molecules which are readily displaced by strongly coordinating reagents.- We have recently been studying the interaction of surface-active and reversibly electroactive functional groups with noble-metal electrocatalysts. Our interest in these systems stems from the fact that chemisorption-induced changes in the electrochemicalproperties of these compounds yield important information concerning the coordination chemistry of the electrode surface. The alteration in the redox potential of a reversibly electroactive species, for instance, is a measure of the surface coordination formation constant of the oxidized state relative to the reduced form. Such behavior is expected to depend upon the composition of the electrode surface. The surface coordination of the iodo ligand at noblemetal electrodes provides an interesting case study since the strong surface activity of this material is expected to bring about profound changes in the redox chemistry of the iodineliodide couple in the surface-bound state. This system is also important since an electrode surface pretreated with a full monolayer of iodine can result in dramatic changes in the kinetics of redox processes of unadsorbed species.s In the present paper, we report recent surface spectroscopic and electrochemical studies of iodine coordinated to the monometals Rh, Pd, Ir, Pt, and Au and the bimetals Ag-Pt and Au-Pt. Among the information sought in this study are (i) the nature of the I-metal surface bond, (ii) the binding strength of the surface-at-
* Author to whom correspondence should be addressed.
Presented at the symposium entitled “Electrocatalysis”, 196th National Meeting of the American Chemical Society, Los Angeles, CA, Sept 27-29,1988.
tached iodine, (iii) the chemisorption-induced changes in the redox potential of the iodine/iodide redox couple, (iv) the influence of surface composition on these redox potentials, and (v) the path of electron transfer from the I-coated electrode surface to solution-bmed electroactive species.
Experimental Section In the present study, the following electrodes were employed single-crystalPd(lll),polycrystalline Rh, Ir, Pt, Au, and Ptl&uso, and Ag-plated Pt. As described previously,’o these electrodeswere annealed at temperatures near their melting points to provide atomically smooth surfaces. The Pd(ll1) electrode was studied by using a custom-built ultra-high-vacuum (UHV) chamber1’ which allowed low-energy electron diffraction (LEED),thermal desorption mass spectrometry (TDMS), and Auger electron spectroscopy (AES) experiments. The other electrodes were studied by using thin-layer electrochemicalcells.@ X-ray photoelectron spectroscopic (XPS) measurements were performed with thin (0.5 mm) foil electrodes. Between experimental trials, the Pd(ll1) surface was cleaned by Ar+ ion bombardment followed by thermal annealing; the polycrystalline electrodes were cleaned simply by sequentialoxidation at potentials just below the oxygen (1) Albert, M. R.; Yates, J. T., Jr. The Surface Scientist’s Guide to Organometallic Chemistry; American Cancer Society: Washington, DC, 1987. (2) Muetteries, E. L. Bull. SOC.Chim. Belg. 1975,84, 959. ( 3 ) Saillard, J. Y.; Hoffman, R. J. Am. Chem. SOC.1984, 106, 2006.
(4) Soriaga, M. P. In Electrochemical Surface Science: Molecular Phenomena at Electrode Surfaces; Soriaga, M. P., Ed.; American Chemical Society: Washington, DC, 1988. (5) Rodriguez, J. F.; Harris, J. E.; Bothwell, M. E.; Mebrahtu, T.; Soriaga, M. P. Inorg. Chim. Acta 1988, 148, 123. (6) Rodriguez, J. F.; Bravo, B. G.; Mebrahtu, T.; Soriaga, M. P. Inorg. Chem. 1987,26, 2760. ( 7 ) Soriaga, M. P.; Binamira-Soriaga,E.; Hubbard, A. T.; Benziger, J. B.; Pang, K.-W. P. Znorg. Chem. 1985,24, 65. (8) Soriaga, M. P.; White, J. H.; Chia, V. K. F.; Song, D.; Hubbard, A. T. Inorg. Chem. 1985,24, 72. (9) Schardt, B. C.; Stickney, J. L.;Stem,D. A.; Frank, D. G.; K a t e h , J. Y.; Rosasco, S. D.; Salaita, G. D.; Soriaga, M. P.; Hubbard, A. T. Inorg. Chem. 1985, 24, 1419. (IO) White, J. H.; Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1984,177, 89. (11) Somorjai, G. A. Chemistry in T w o Dimensions: Surfaces; Cornell University Press: Ithaca, 1981.
0743-746318912405-070~$01.50/0 0 1989 American Chemical Society
708 Langmuir, Vol. 5, No. 3, 1989
Berry et al. J
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Figure 1. Thin-layer cyclic current-potential curves for hydroquinone (HQ) at smooth polycrystalline Rh, Pd, Ir, and Pt wctrodes before and after surface pretreatment with iodine. The solutions contained 1mM HQ in 1 M NaClO, buffered at pH 4. Volume of the thin-layer cells, V = -4.0 pL. Areas of the electrode: ARh = 1.10cm2;Apd = 1.08 cm2;AI, = 1.25 cm2;AR = 1.14 cm2. Potential sweep rate, r = 2 mV/s. Temperature, T = 298 K.
evolution region and reduction at potentials within the hydrogen evolution region in 1 M H2SOI. Electrode potentials reported here were referenced against a Ag/AgCl (1M Cl-) electrode; all solutions were prepared with pyrolyticallytriply distilled water.'* Chemisorption of iodine was accomplished (i) by immersing the clean electrode in an aqueous solution of 12 or KI or (ii) by vapor dosing with I, or HI. Vapor dosing in UHV was done at room temperature, but I2 dosing under ambient pressures was carried out at 700 "C in a furnace through which a slow stream of Nzgas saturated with Izvapor was passed.I3 For the Pd(ll1) electrode, I coverages were measured by LEED and AES, as described elsewhere." For the thin-layer electrodes,the absolute packing density of iodine, rI,waa determined by anodic oxidation of the surface iodine to aqueous iodate followed by coulometric measurement of the charge, (Q- Qb)I@-, needed to reduce Io3-(aq) to Iz(aq):I6
rr= (Q- Qh)1OS-/5FA
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where F is the Faraday constant, Qb is the background charge measured in the absence of surface iodine, and A is the surface area. Surface area determinationsfor the polycrystallineelectrodes were based upon underpotential hydrogen depositionloor iodine chemisorpti~n.~~ LEED and AES experimentswere carried out with PHI 15-120 and 10-155components, respectively, mounted on a custom-built UHV chamber" operating at a base pressure of less than 5 X lO-'O (12)Conway, B. E.;Angerstein-Kozlowska, H.; Sharp, W. B. A.; Criddle, E. E. Anal. Chern. 1973,45, 1331. (13)Wieckowski, A.; Rosasco, 9. D.; Schardt, B. C.; Stickney, J. L.; Hubbard. A. T.Inorp. Chern. 1984. 23. 565. (14) Stickney,J. Ly;Roeasco, S. D.;sbng,D.; Soriaga,M. P.; Hubbard,
A. T. Surf.SCL.1983, 130, 326. (15) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J . Electroanal. Chem. 1987,233, 283.
Torr. XPS spectra were obtained on an HP 5950A spectrometer using Al Ka radiation at an incident angle of 38". The resolution of this instrument is f 0 . 5 eV.
Results and Discussion Figure 1 shows thin-layer cyclic current-potential curves for the two-electron, two-proton quinone (Q)-to-hydroquinone (HQ) redox process of a 1 mM aqueous solution of hydroquinone in 1 M NaC104phosphate-buffereda t pH 4 a t Rh, Pd, Ir, and Pt surfaces before and after coating with a full monolayer of iodine. It will be mentioned that for a thin-layer electrochemical experiment the anodicto-cathodic peak separation, AE = *Ep- $,,, is a direct measure of Ito, the electron-transfer rate constant? log ko = -(1260/7')(AE) + log (VF(rl/(2ART)) ( 2 ) where T is the absolute temperature, V the thin-layer volume, r the potential sweep rate, and R the gas constant. Under thin-layer conditions, for totally irreversible (ko< 10+ cm s-l) redox couples, AE > 200 mV; for very rapid (ito> cm s-l) reactions, AE vanishes.16 It can be seen that, at Rh, Pd, Ir, and Pt, pretreatment of the electrode surface with a full monolayer of iodine results in significant enhancement of the kinetics of the Q(aq)/HQ(aq) reaction. Table I lists the measured AE values for Rh, Pd, Ir, Pt, and Au electrodes; these values have been corrected for thin-layer ohmic drop by using an Fe(III)/Fe(II) couple.16 As can be seen in this table, the Q(aq)/HQ(aq) reaction rate a t the untreated electrode decreases in the order Au (16) Hubbard, A. T. Crit. Rev. Anal. Chern. 1973, 3, 201.
IodineINoble-Metal Electrode Surfaces Table 1. Electrode Kinetic Parameters of the Benzoquinone/ Hydroquinone Couple at Noble-Metal Electrodes before and after Pretreatment with Iodine electrode rhodium I-coated Rh palladium I-coated Pd iridium I-coated Ir platinum I-coated Pt gold I-coated Au
(.Ep - &E,)/mV 35 15 50 25 100 25 15 25 10 15
'The erperimenta were performed by using 1 mM hydroquinone in 1 M NaCIO, buffered at pH 4. ,Ep is the anodic peak potential, CEp is the cathodic peak potentid. The values have heen corrected for ohmic drop. Other experimental conditions were as in Figure 1.
> Rh > Pd > Pt > Ir. It has to be pointed out that, in the absence of an I coating, the Rh, Pd, Pt, and Ir surfaces contain a monolayer of irreversibly adsorbed HQ-derived intermediates; no such chemisorption occurs on Au." Iodine pretreatment dramatically increases the reaction rate at Ir and Pt electrodes; enhancement, albeit slight, is also observed at the I-coated Rh and Pd. In contrast, rate retardation is brought about by I treatment at the Au surface. Since HQ is not chemisorbed on Au and the rate on the untreated Au electrode is the highest among the surfaces studied, it may be argued that the presence of HQ-derived surface intermediates at Rh, Pd, Ir, and Pt retards the reaction rate probably because the organic layer acts as an insulator and depopulates the surface sites available for electron transfer. I pretreatment of the Rh, Pd, Ir, and Pt surfaces prevents the formation of the insulating organic layer, and, since the I layer is a better electron conduetor than the organic layer, it enhances the electrode kinetics of the Q/HQ reaction. The relative conductivity of the I monolayer can be gleaned from the data for clean and I-treated Au: the I-coated Au is only slightly less conductive than the uncontaminated Au. In trying to understand the electron-transfer characteristics of the I-coated surface, it is important to determine the nature of the I-metal sixface chemical bond. For example, it is critical to ascertain whether the surface species is ionic or neutral since the ionicity of the surface species has important ramifications in the electron-transfer properties of the I-metal interface. The following approaches have been taken to address the question of the valency of the surface iodine species: (i) determination of the number of electrons transferred in the oxidative desorption of the surface iodine to aqueous iodate (for example, if an n value of 5 is measured, it means that the surface species is zero valent), (ii) investigation of the reductive desorption of surface iodine, (iii) comparison of the surface structure and composition of species derived from molecular iodine with that obtained from HI or aqueous iodine salts,(iv) comparison of the XF'S chemical shifts of the I-metal interface with those for molecular iodine, iodine salts, iodate salts, zero-valent metal, and high-valent metal compounds. Results from studies of the number of electrons transferred during the oxidative desorption of surface iodine (17) Bravo, B. G.;Mebrahtu. T.; Soriaga, M. P. J . E l e e t m ~ IChem. . 1988,241,199, (18) Berry, G. M.; Bothwell, M. E.; Bravo. 9. G.; Cali, G. J.; Harria, J. E.; Mebrahtu, T.; Michelhaugh, S. L.; Rodriguez, J. F.; Soriaga, M. P. Longmuir, to be submitted.
Langmuir, Vol. 5, No. 3, 1989 709
on Ir, Pt, and Au have been reported These studies have shown that oxidation from I(ads) to IO EB > 618.0 eV. It is most interesting to note that XPS measurements for Fe(100)-I(ads)2sand Ag(lll)-I(ads)n also lead to the same conclusion concerning the zero valency of I in the adsorbed state. The zero valency of both surface metal and adsorbed I atoms can only mean that iodine is covalently bonded to the metal surface; that is, exceedingly little ionic character exists in the chemical interaction between I and the metal surface. A theoretical treatment to describe the covalent bonding between the metal surface has not been attempted. One approach would be to deconvolute the
712 Langmuir, Vol. 5, No. 3, 1989
Berry et al.
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surface band structure into components of given symmetries with respect to a given ~ i t e . l -In ~ this manner, the surface-adsorbate bond can be described in terms of the overlap of symmetry-adapted surface bands and adsorbate orbitals, a description exactly the same as that of the metal-ligand bond in monometal or small cluster complexes. In contemplating the nature of the I-metal interface, it is important to recognize that there is no bonding interaction between the close-packed surface I atoms. In contrast to the delocalized density of states a t the pure metal surface, there is a high degree of site localization of the density of states a t the I-metal interface; the density of states is expected to be highest a t sites where I atoms are located. Evidence for this may be gleaned from scanning tunneling microscopy (STM) experiments%which (28) Szklarczyk, M. Private Communication.
show changes in tunneling current, manifestations of changes in the surface local density of states, when I is chemisorbed onto an atomically smooth Au(ll1) surface. A further consequence of the covalent nature of the Imetal interface is that electron transfer has to emanate from the I adatom and not from pure metal sites. The conductive nature of the I adatoms is also indicated in STM experiments:2s no tunneling currents would have been observed on an I-Au surface if the I sites were nonconducting. The fact that the surface electrochemical properties of iodine are profoundly dependent on the composition of the electrode surface makes it possible to employ iodine chemisorption as an electrochemical probe of the interfacial and electrocatalytic properties of mixed4metal surfaces. We have employed this approach to investigate the interfacial properties of a silver-plated Pt electrode surface.
Langmuir, Vol. 5, No. 3,1989 713
Iodine/Noble-Metal Electrode Surfaces
Example results from this investigation, in terms of the potential and pH dependence of the I coverage as the amount of electrodeposited Ag is varied, are given in Figure 6. The upper part of this figure illustrates the difference between pure Pt and bulk Ag plated Pt in the 6, vs E plots, where BI is the fractional coverage of iodine defined by rI/r, Clearly, the 0, vs E data for the Ag-plated surface are pH-independent, as expected since bulk Ag does not dissociatively chemisorb hydrogen as pure Pt does. The bottom portion of Figure 6 shows BI vs E curves when only half-a-monolayer of Ag is present on the Pt surface. When the top and bottom parts of Figure 6 are compared, it can be seen that two portions appear in the BI vs E plots for the Pt surface plated with 0.5 layer of Ag: one, in the region from BI = 1to BI 0.5, obeys the same pH dependence and steep slope of the plots when no Ag is codeposited; the other, in the region BI < 0.5, is pH independent and characterized by a less steep slope. These results suggest that, a t least a t the negative potentials where I desorption occurs, iodine is bonded to two different surface sites: those on Pt sites give rise to the pH-dependent behavior, and those on Ag sites account for the pH-independent character.
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Figure 7. Fractional coverage of iodine, 01, plotted as a function of potential at pH 0 (1 M H2S04),7 (phosphate-buffered 1 M NaC104),and 10 (carbonatebuffered 1M NaCI03 for I-pretreated monometal Au and bimetallic Pt,&u, electrodes. The solid lines interconnectthe data ints and do not represent any theoretical fit. APt-Au= 1.08 cmP.O Other experimental conditions were as in Figure 1.
redox potential for the 12(aq)/I-(aq)couple, is a measure of the relative chemisorption strengths of zero-valent iodine and iodide which, in turn, can be used to calculate the ratio of the respective surface-complex formation constants Kf,I/Kf,,-. The present data yield Kf,I/Kf,r- lo3$ in comparison, the corresponding values for Ir,20Pt,21and A u are ~ ~ and loB, respectively. We have also studied the interfacial electrochemical properties of Pt-Au bimetallic alloys by observing the changes, or absence thereof, in the iodine adsorption-desorption characteristics in going from monometal to mixed-metal surfaces; an example result is shown in Figure 7 . In this figure, BI vs E plots at pH 0 , 7 , and 10 are given for pure Au and a Ptl&uw bimetallic catalyst. The conclusion which one obtains from these data is that, insofar as the reductive desorption of iodine is concerned, the Ptl&uw bimetallic alloy behaves the same as a pure Au electrode. This finding is not unexpected in view of the facts: (i) I is more strongly chemisorbed on Au than on Pt,21,22(ii) a t least 90% of the surface sites is Au atoms (there is little reason to suggest accumulation of Pt on the surface since I prefers Au to Pt), and (iii) an iodine atom is approximately2 times as large as either a Pt or Au atom. The results depicted in Figure 7 also demonstrate that the surface of a Ptl,,Auw alloy is homogeneous. If the surface atoms were aggregated as Pt and Au islands, a noticeable difference in the BI vs E plots for pure Au and Ptl&uw alloy would have been observed similar to that in Figure 6 for the Ag-plated Pt interface.
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The result that the pH-dependent portion of the BI vs E plots terminates when BI 0.5 is significant since it can be shown from surface coverage calculations that, if the half-monolayer Ag is aggregated as islands, then half of the chemisorbed iodine would lie on top of the Ag domain (I'I*Ag/I'I,= 0.5) while the other half remains on the Pt substrate (I'I,pt,I'r,mu= 0.5). From the upper portion of Figure 6, one can identify &(ads), the redox potential for the I(ads)/I-(ads) surface reaction for a bulk Ag-plated Pt surface, with the potential at which the iodine coverage is half-maximum, -1.1 V. The difference EOI(ads)- E$(aq), where the latter term is the
Acknowledgment is made to the Robert A. Welch Foundation, to the Texas Engineering Experiment Station, and to the Regents of Texas A&M University through the AUF-sponsored Materials Science and Engineering Programs for support of this research.
